The optical network construction has been put to practical use by a wavelength division multiplexing (WDM) transmission technique. Recently, the WDM transmission device has been applied not only to a basic network (backbone network) of a large capacity, but also to a relatively small network (metro network, access network, etc.).
Low cost equipment is especially demanded in the above-mentioned small network. However, there is a case in which the transmission function is limited by deleting some functions by cost reduction.
FIG. 1 is an example of a configuration of a network by a WDM transmission device.
The network is configured by a transmitting terminal station 11 in which transmitters 10-1 through 10-n are arranged, a receiving terminal station 12 in which receivers 13-1 through 13-n are arranged, and a relay station 15, which is provided with an inline optical amplifier connected by a transmission line 14 of optical fiber, between the transmitting terminal station 11 and the receiving terminal station 12.
Generally, in the WDM transmission system, it is requested to exchange information between stations. There are methods proposed for transmitting various types of information between the stations.
For example, there is a method of preparing an optical supervisory channel (OSC) for monitor control in addition to a main signal, multiplexing the channel to the main signal, and transmitting a resultant signal (refer to Patent Documents 2 and 3).
The transmitters 10-1 through 10-n transmit the signal light of different wavelengths λ1 through λn, and the signal light is multiplexed by a multiplexer 18. Furthermore, the OSC signal from a transmitter 17 of optical supervisory channel (OSC) for monitor control is multiplexed to the signal light of wavelengths λ1 through λn, and transmitted as WDM signal light from the transmitting terminal station 11. The WDM signal light from the transmitting terminal station 11 propagates the transmission line 14 configured by optical fiber, and is input to the transmission line 14. The transmission line 14 receives the OSC signal by an OSC receiver, processes the OSC signal, transmits the signal by the OSC receiver, amplifies the main signal by the optical amplifier, and transfers the signal. The receiving terminal station 12 extracts the OSC signal from the WDM signal light received from the transmission line 14, the signal is received by an OSC receiver 16, and the main signal is input to a demultiplexer 19. The demultiplexer 19 demultiplexes the WDM signal light to signal light of each wavelength, inputs the light to the receivers 13-1 through 13-n for the respective wavelengths, and a signal is output by the receivers 13-1 through 13-n. 
The OSC is received by the OSC receiver 16 in each station and opto-electriccally converted, and then requested information is retrieved. Then, it is electro-optically converted by the transmitter 17 for the OSC, and transmitted to the next station.
An example of the information exchanged between the stations is described below.
each type of command for remotely controlling a device or acquiring requested information
information about presence/absence of an issue of a warning
various types of light feature information such as light intensity input to a device, output power of an optical amplifier, etc.
number of wavelengths of light being processed
Especially, the number of wavelengths of light being processed is used for control of an optical amplifier in a wavelength division multiplexing transmission device, transmitted to each downstream station by OSC etc., and used in control of an optical variable attenuator for correcting the gain of the optical amplifier, and adjusting the input power of the optical amplifier.
In the wavelength division multiplexing transmission system, an optical amplifier of rare-earth-doped fiber such as erbium etc., a semiconductor optical amplifier, etc. are often used as an optical amplifier.
With the above-mentioned optical amplifier, the amplifier itself generates noise light when signal light is amplified. It is known as amplified spontaneous emission (ASE).
The gain of the optical amplifier is controlled by arranging a detector using a photodiode etc. to detect light intensity in the input unit and the output unit of the optical amplifier, and detecting the ratio of the light intensity.
However, the detector of the light intensity does not discriminate the signal light from the ASE generated by the optical amplifier, but the detected light intensity includes both signal light and ASE. Based on the light intensity information including the ASE, the gain of the optical amplifier is controlled. Therefore, when the ratio of the intensity of the signal light in the total light intensity is low, the effective gain to the signal light is degraded.
The low ratio of the intensity of the signal light in the light intensity is especially outstanding when the number of wavelengths is smaller.
FIG. 2 is an example of calculating the amount of gain reduction by the ASE.
In the calculation in the graph illustrated in FIG. 2, the following conditions are set.
wavelength1547nmgain of optical amplifier20dBnoise coefficient of optical amplifier5dBASE generation band width30nmintensity of output power of optical amplifier (par wave)0dBmloss between relay stations20dB
FIG. 2 illustrates the case in which the number of inline optical amplifiers cascade-connected by the transmission line is 1 through 3 stages. In each case, as the number of wavelengths becomes larger, the amount of gain reduction of the signal light becomes smaller. That is, when the number of wavelengths is small, the amount of gain reduction of the signal light becomes large.
Thus, with the gain of the optical amplifier effectively reduced with respect to the signal light, the lower optical SN ratio and the lower level of receiver input may occur, thereby degrading the transmission performance.
There are some methods of correcting the reduction of the gain to the signal light (refer to the patent document 4).
In the method of the patent document 4, the amount of correction (amount of the added gain) of the requested gain is determined by calculation or from a prepared correction amount table.
When the amount of correction of gain is determined, the wavelength number information is requested. The wavelength number information is normally acquired at the transmitting terminal station, and transmitted to the inline amplifier (relay station) arranged downstream.
Some other conventional techniques (for example, patent document 1) may detect the number of wavelengths depending on the input light level of an input optical signal.